Such a test piece is known from DE 199 15 012.
Test pieces serve in general for the adjustment and the surveillance of specially positioning and measuring systems, in particular of mobile coordinate measuring systems. They customary consist of reference elements, so called shaped probe elements, which are connected by connecting elements and which have accurately defined distances from each other. A coordinate measuring system is checked by determining the relative positions of the shaped probe elements. The relative positions obtained in that way are compared with the actual distances of the test piece.
Test pieces should fulfill various requirements. At the one hand the properties of a test piece under standard measuring conditions, i.e. at temperatures between −20° Celsius and +70° Celsius and a humidity between 0% and 100%, should not or just insignificantly vary. In particular, this means that the relative positions of the shaped probe elements should not vary significantly with changes in temperature or in humidity under standard measuring conditions. At the other hand a test piece should be transportable with economically justifiable effort, so that is suitable for the surveillance of mobile devices. Therefore, a test piece should not show a high self-weight and should be easily mountable and demountable.
The test piece described in DE 199 15 012 comprises ball-shaped probe elements and rod-shaped connecting elements. The shaped probe elements and the connecting elements altogether form a tetrahedron, at the corners of which the shaped probe elements are arranged. The material and/or the dimensioning of the shaped probe elements and the material and/or the dimensioning of the connecting elements are matched to each other in such a way, that the distance between two respective sensing points essentially does not change under standard measuring conditions. This is achieved thereby, that the shaped probe elements comprise a material with a positive or a negative length expansion coefficient and the connecting elements comprise a material with a negative or a positive length expansion coefficient, respectively, under standard measuring conditions, such that the respective length contractions or length dilations are compensated. The connecting elements consist for example of carbon fibre composite material or glass-ceramic material.
A further test piece is known from EP 0 350 532. Therein, the connecting elements consist of a fibre composite material, in which layer of circumferential windings and cross windings alternate, wherein the cross windings are wound with one fibre, that has negative thermal expansion coefficient. Whereas the balls consist of a material which has a positive thermal expansion coefficient.
A first disadvantage of the mentioned prior art is, that the material and/or the dimensioning of the complete connecting elements have to be chosen such that the expansions or contractions of the shaped probe elements are compensated. Thus, one is strongly restricted in the choices of the material and in particular in the dimensioning of the connecting elements. A further disadvantage is that special materials with negative thermal expansion coefficients are generally more expensive to produce, from which a high price of the test element results.
The problem underlying the invention is therefore to overcome said disadvantages and to provide a test piece, which is particularly suited to accurately check mobile coordinate measuring machines, but in which the material and/or the dimensioning of the connecting elements do not have to be chosen primarily in dependence of the material and/or the dimensioning of the shaped probe elements.